Answer: I garden in the hot Southwest, so I understand your issue. These annuals take heat and sun and offer lots of variety in colors and sizes: sunflowers, zinnia, cosmos, gaillardia, petunia, salvia, vinca, tithonia, black-eyed Susan, portulaca, celosia, globe amaranth, verbena, four o?clock, and lisianthus.
Plastic containers retain soil moisture better than porous materials, such as clay. Also, use a light color pot, which won't absorb as much sun/heat as a dark pot. If you have decorative clay and/or dark containers that you like, try planting in a slightly smaller plastic pot and set it inside the other pot.
Without knowing what was in your soil mix and what the specific issue was, it's hard to provide an alternative. However, in general, good drainage is essential for a container soil mix. I also find that peat moss, once it dries out, is difficult to rewet, so it can be hard to keep the entire root ball moist--water flows down the sides of the pot and out the drainage holes. Peat moss is often a key ingredient in soil mixes. You might seek one that doesn't have it, or at least doesn't contain alot of it.
In really hot conditions, it may be necessary to water containers daily. More frequent feeding is also required because the frequent watering acts to wash away nutrients.
This report on container gardening has a few other hints for keeping pots cool in direct sun:
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